EIXVIER Journal of Magnetism and Magnetic Materials 172 (1997) 61-68
Magnetic and Miissbauer investigations in sputtered Fe/Pt multilayers
H. Law-P*, M. Abidb, R. Krishnanc, A. Fnidikid, J. Teilletd
a Laboratoire de Physique des Mat&iaux et de Microklectronique, Universiti Hassan II, Faculti des Sciences, B.P. 5366, A% Chock, Route d’El Jadida km-& Casablanca, Morocco
b Laboratoire de Physique Thioorique, Universiti Hassan II, Faculth des Sciences, B.P. 5366, Ai’n Chock, Route d %I Jadida km-i?, Casablanca, Morocco
’ Laboratoire de Magnftisme et d’optique de l’llniversitc? de Versailles, CNRS, URA 1531, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France
’ LMA. URA CNRS 808, Fact& des sciences de Rouen, 76821 Mont-Saint-Aignan Cedex, France
Received 11 November 1996; received in revised form 18 February 1997
Abstract
Fe/Pt multilayers with varying thicknesses of Fe and Pt layers were prepared by RF diode sputtering and their magnetic properties in the range 5-295 K were studied. Conversion electron MBssbauer spectra were also taken at 295 K.
From the analysis of the magnetization dependence on Fe layer thickness, the polarization induced moment of Pt atoms is estimated to be 0.5 pe at 5 K. The interlayer coupling remains ferromagnetic independent of the Pt layer thickness.
Interdiffusion at the interface is evidenced by the distribution in hyperfine fields.
Keywords: Multilayers; M&sbauer spectroscopy; Interlayer coupling; Perpendicular anisotropy; Magnetization
1. Introduction
Recently, multilayer films composed of Co and noble metals have attracted much attention as can- didates for high-density magneto-optical infor- mation storage. Co/Pt [l-4] and (Co-Ni)/Pt [5]
multilayers are now well-known examples. These artificial multilayered structures are generally based on layer stacking along the (1 1 1) Pt direc-
tion. Polycrystalline Pt 1 _xFe, and Pt I _XCo, alloys, some possessing perpendicular anisotropy, have also been prepared and suggested for mag- neto-optic information storage [6,7]. In this paper, we report some magnetic and Miissbauer proper- ties of a series of Fe/Pt multilayers prepared by RF diode sputtering.
2. Experimental details
* Corresponding author. Fax: + 212 2 230674.
The multilayers were deposited onto water- cooled glass substrates by RF diode sputtering. The 0304-8853/97/%17.00 0 1997 Elsevier Science B.V. All rights reserved
PII SO304-8853(97)00097-S
chamber was first evacuated to a pressure of l-2 x lo-’ T using a turbomolecular pump fol- lowed by overnight baking. Argon at a constant pressure of 6 x 10v3 T was used as a sputter gas.
The RF power density was 2 W/cm’. The thickness was measured in situ using a pre-calibrated quartz monitor. All the samples were grown on a Pt buffer layer 100 A thick. In one series, the magnetic layer thickness tFe was varied in the range 2.5-60 A and that of the Pt layer tp, was fixed at 18 A. In the second series, tFe was fixed at 24 A and tp, was varied in the range 7-50 A. The second series was to check if there was any modification in the nature of coupling between magnetic layers when tp, was changed.
Low- and high-angle X-ray diffraction studies were made to verify the periodic structure and to calculate the layer thickness.
Magnetization and M-H loops were measured in the range 5-295 K, with a vibrating sample mag- netometer (VSM). We carried out conversion elec- tron Mijssbauer spectroscopy (CEMS) studies at 295 K using a conventional spectrometer equipped with a home-made helium proportional counter.
For fitting the spectra we used the histogram
method, constraining the width of each elementary spectrum to be the same.
3. Results and discussion
The structure of the films was examined by X-ray diffraction using CoKol radiation in both low- and high-angle regions. Low-angle X-ray diffraction of all the samples revealed peaks typical of the modulated structure, and the thickness calculated from these peaks agrees within 3% with that ob- tained from the quartz oscillator after calibration.
Fig. la and Fig. lb show the low-Oand high-angle X-ray diffractions for a sample (23 A Fe/18 A Pt)32.
The (1 1 1) peak from the Pt buffer layer and the satellite peaks are clearly seen in Fig. lb.
Figs. 2 and 3 show the dependence of magne- tization M of Fe/Pt multilayers on the thickness of the Fe layers, tFe, and of the Pt layers, tpt, respect- ively, measured by VSM. For constant tpt, the mag- netization M first increases with decreasing tFe and then starts decreasing after showing a peak. The decrease of M for very small values of tFe can be understood as due to a two-dimensional effect [S].
b
I I I I I, I I I I I
-$++-&+’ 42 44 46 48 50 52
20 0%)
Fig. 1. The low- (a) and high-angle(b) X-ray diffraction patterns for the sample (Fe 24 &Pt 18 A),, deposited on 100 A thick Pt buffer layer. The peaks marked (open circle) correspond to the disordered interface phase and the peaks marked (close circle) correspond to a-Fe.
H. Lassri et al. /Journal of Magnetism and Magnetic Materials I72 (1997) 61-68 63
2500
1000
500
0
*
0 20 40 60
tFe ( i >
Fig. 2. The tFe dependence of the magnetization at 295 and 5 K.
1500
1250
-x.x .
. \ ,, \ \ Bulk
\X \
\ \ . . x.--x-
+24 A
l-=295 K
I 1
0 15 30
80
60
Fig. 3. The tp, dependence of the magnetization at 295 K.
The increase in A4 observed for a certain range of fFe can be explained, as is well known, either by an enhancement of the magnetic moment of Fe atoms at the interface or by the moment of Pt layer in- duced by polarization effects from Fe. When tFe is kept constant and tp, is varied, M shows an in$rease above that of the bulk value, for or, 2 15 A, but it starts decreasing for tp, 3 30 A. Finally, for tp, > 45 A M reaches a stable value. The result is shown in Fig. 3. The increase in thin Pt layers arises from the fact that in these samples the inter- face is no longer continuous and, therefore, there are more Pt neighbours around Fe atoms near the interface. This leads to an increase in the number of Pt atoms which are polarized by Fe atoms. For thicker Pt layers, the interface being continuous, only those Pt atoms which are nearest to the Fe layer get polarized and this is a constant. So
M reaches a plateau. From a seties of samples of constant tp, = 18 A and tFe > 20 A, we find a linear relation between Mt,, and tFe (Fig. 4) from which
we have deduced the polarization of Pt and the magnetization of the Fe layers. At 5 K the magnetic moment of the Pt atoms is estimated to be 0.5 mB, and the magnetization of the Fe layers is about 1760emu/cm3, the latter is in agreement with the bulk value for Fe.
Let us discuss the effect of varying tp, on the nature of coupling at 295 K. We studied the in- plane M-H loop! of the samples with tFe = 24 A and 7 < tp, < 50 A, in order to study the variation of the saturation field and remanence ratio r = M,/M, as a function of tpt. For ferromagnetic coupling, the saturation field is expected to be lower than the case when the coupling becomes antiferromagnetic for certain tpt, when it would increase rapidly. Our study shows that the in-plane loops are quasi-rectangular with signs of aniso- tropy in the film plane. The saturation field is smaller than 50 Oe and remanence ratio is about 0.8 indicating that the interlayer coupling remains ferromagnetic for the various tp, studied here.
120
0 25 50
tF+ ( A >
Fig. 4. The tFe dependence of the product Mt,, at 5 and 295 K.
tn=1s b:
XxXxX T&295 K
***WT=5 K
75
H. Lassri et al. /Journal of Magnetism and Magnetic Materials I72 (1997) 61-68 65
Fig. 5 shows both the parallel and perpendicular M-H loops for the sample with tFe = 2.5 A (tpt = 18 A) at 5 K. The approximate isotropy could be explained by a perpendicular anisotropy, which roughly cancels the shape anisotropy.
Fig. 6 shows the temperature dependence of M for several values of tFe thicknesses. It can be
H(kOe)
Fig. 5. In-plane and perpendicular M-H loops for the sample with tFc = 2.5 A at 5 K.
2500
noticed that Tc decreases when tFe decreases as is to be expected due to a decrease in the exchange interaction arising from less Fe neighbours. The low-temperature magnetization was studied in detail for a few samples. For three-dimensional magnetic films, the magnetization has a T3”
dependence due to the classical spin-wave exci- tations. In such cases, according to spin-wave theory, the temperature dependence should follow the relation
AMGYM, = CM, - W’W~o
= BT312 + CT5”. (1)
For Fe layers as thin as 10 A the magnetization exhibits such a T 3’2 dependence as shown in Fig. 7.
In all cases, this behaviour is observed for temper- atures as high as Tc/3. The spin-wave constant
B was calculated and it is found to decrease from 47 x lop6 to a value of 7.3 x 10e6 KW3” when tFe increases from 8.5 to 57 A. These values are much
-+- -St-+--7
===---
***** t357.0 _ _ Bulk
0 50 100 150 200
T
(K)
300
Fig. 6. Temperature dependence of the magnetization of Fe/Pt multilayers with varying Fe thicknesses.
2250
1500
1250
tR=la A
+wi+ tr*= 8.4 K XXXXX bp24.0
***** b*=57.0 AMM Bulk
0 2000 4000 6000
T3/3 ( K3/2)
>
‘* \
* tR=18 ii
T \*
\’ 0
0 \ t,.-2.5 A
\
” 300 *
Lc
‘t
\ \
*\
L \
Fig. 7. M(T) vs. T3j2 for Fe/Pt multilayers for various Fe thicknesses. The slopes of the fitted lines give the spin wave constant B.
450
\
150 *\ \
\
*\
w \
‘* \
\
C 0 100 200 300
‘I’ (K)
Fig. 8. Temperature dependence of the Hc for Fe/Pt multilayer with tFc = 2.5 A.
H. Lassri et ai. /Journal of Magnetism and Magnetic Materials 172 (1997) 61-68 61
larger than the value of 5 x lO-‘j K-3’2 found for bulk Fe, which again indicates the decrease in the
which results from two-dimensional spin-wave exchange interaction for thin Fe layers. However,
excitations and the island structure of the layer for ,,the thinnest Fe layer sample (Fe 2.5 &Pt C9,lOl.
18 A)22, the temperature dependence of demagne- Fig. 8 shows the temperature dependence of tization, shows a linear behaviour as shown in Fig. 6
Hc of the sample (Fe 2.5 &Pt 18 A),,. It can be seen that Hc decreases linearly with increasing
-2 0 +2
1.01
vdocity (mdf4
-10 0 +lO
1.01
IYLd!cL tF,=l2 ;a ’ 1.00
1.01
PO-V
3. P(H) 25 20 15 10 5
l.“” o-m-$-m”&-_
20 30
tFe’36 di H(T)
Fig. 9. CEM! spectra at roSom temperature and hyperfme-field distributions of some Fe/Pt multilayers (tp, = 18 A). (a) rFe = 2.5 ,&;
(b) tFe = 12 A; (c) tFe = 15 A; (d) tFe = 20 A; (e) tFc = 36 A.
temperature which is also somewhat similar to that of M with temperature.
CEMS spectra of some Fe/Pt multilayers are shown in Fig. 9. For the sample with the thinnest Fe layer of 2.5 A, the spectrum is a slightly asym- metrical doublet with broad lines (line width G = 0.71 mm s- ‘). This broadening may be at- tributed to the weak magnetic state with mean hyperfine field of about 3 T. For samples with thicker Fe layers, the slight asymmetrical magnetic spectra with broad lines are related to a distribu- tion in the environments of Pt and Fe. For tFe = 12 0 or 15 A, the continuous hyperfine-field distribution (Fig. 9) may be attributed to a magnetic interface (Fe-Pt) phase which is probably compositioaally modulated in the layer. For tFe = 20 and 36 A the hyperfine-field distribution is the sum of a distribu- tion analogous to the previous one, characteristic of modulated interface and of a narrower distri- bution centred on 33 T, characteristic of the bulk cubic a-Fe. This agrees with our X-ray diffraction results and with the appearance of a-Fe from tFe close to 20 A as already observed in this system [ll]. This indicates that the thickness of Fe layer is then enough for the centre of the Fe layer to be constituted of pure Fe. The relative intensity of a-Fe contribution increases with tFe leading to an increase of the mean hyperfine field with tFe. At room temperature, for the samples with tFe B 12 A, the magnetization was found to be in the plane which is due to the dominant effect of the demag- netization energy.
4. Conclusions
In conclusion, we have prepared Fe/Pt multi- layers by sputter deposition and studied their magnetic properties. In the (Fe 2.5 @Pt 18 A),,, multilayer, the approximate isotropy could be ex- plained by a perpendicular anisotropy which roughly cancels the shape anisotropy. The interlayer coupling has been found to remain ferromagnetic independent of Pt layer thickness. The Curie temperature was found to decrease with decreasing Fe layer thickness.
References
Cl1
c21
c31 141 t-51 C61 c71 PI l-91 Cl01 IIll1
P.F. Garcia, J. Appl. Phys. 63 (1988) 5066.
W.B. Zeper, F.J.A. Greidanus, P.F. Garcia, C.R. Fincher, J. Appl. Phys. 65 (1989) 4971.
Y. Ochiai, S. Hashimoto, K. Aso, Jpn. J. Appl. Phys. 28L (1989) 659.
R. Krishnan, M. Porte, M. Tessier, IEEE Trans. Magn. 26 (1989) 2727.
R. Krishnan, H. Lassri, M. Seddat, M. Porte, M. Tessier, Appl. Phys. Lett. 64 (1994) 2312.
D. Treves, J.T. Jacobs, E. Sawatzky, J. Appl. Phys. 46 (1975) 2760.
T.R. McGuire, J.A. Aboaf, E. Klokholm, J. Appl. Phys. 55 (1984) 1951.
M.R. Khan, P. Roach, I.K. Schuller, Thin Solid Films 122 (1984) 183.
C.J. Gutierrez, Z.Q. Qiu, M.D. Wieczorek, H. Tang, J.C.
Walker, R.C. Mercader, Hyperfine Interactions 66 (1991) 299.
Z.Q. Qiu, S.H. Mayer, C.J. Gutierrez, H. Tang, J.C.
Walker, Phys. Rev. Lett. 63 (1989) 1649.
A. Fnidiki, J.S. Douheret, J. Teillet, H. Lassri, R. Krishnan, J. Magn. Magn. Mater. 156 (1996) 41.
